kthix10 hits the nail on the head below for the basics. Becoming a CFP, Certified Financial Planner, requires different levels of training and certification depending on where you are on the planet. But that still is only part of the answer.
Genuine interest in performance and an understanding of 1. the clients needs and 2. the market and how it works are what grounds the greatest performers.
People hire people to handle there money for a few different reasons. Sometimes they don't trust themselves with the money and somehow feel that someone else can be trusted more than themselves. Sometimes it is because they know the other guy/gal MUST know more about money than they do, or sometimes they are just either bored with or not interested in learning about what makes that kind of thing work.
Whatever the motivation, trust, honesty and integrity are key to being the person who should succeed for all concerned.